Transport refrigeration systems for conditioning the loads of trucks, trailers and containers have cooling, null and heating modes. The heating mode includes a heating cycle for controlling load temperature to a selected set point, as well as a heating cycle for defrosting the evaporator coil. When the system switches from a cooling or null mode into a heating cycle, hot compressor discharge gas from a refrigerant compressor is diverted by suitable mode selecting valve means from a cooling cycle refrigerant path, which includes a condenser coil, a receiver tank, an expansion valve, an evaporator coil, and an accumulator, to a heating cycle refrigerant path which includes the compressor, the evaporator coil, and the accumulator.
U.S. Pat. No. 3,370,438 teaches reducing the active size of a condenser coil during low ambient conditions, with the portion of the condenser coil cut out of the active system having a drain line which drains refrigerant from the inactive portion of the condenser coil into the active system.
To make more liquid refrigerant available during a heating cycle, a prior art procedure pressurizes the receiver tank with hot compressor discharge gas to force liquid refrigerant out of the receiver tank and into the refrigerant cooling circuit. This requires an auxiliary hot gas line which runs from the main hot gas line to the receiver tank, along with a by-pass check valve, a by-pass service valve, a receiver tank pressure solenoid, and a condenser check valve. A bleed port in the expansion valve allows the liquid refrigerant forced out of the receiver tank to flow into the evaporator coil during the heating cycle, to improve heating and defrosting capacity.
U.S. Patent No. 4,748,818, which is assigned to the same assignee as the present application, improved upon the aforesaid prior art procedure by connecting the output of the receiver tank to the accumulator during a heating cycle. This eliminated the auxiliary hot gas pressure line to the receiver tank, and the hereinbefore mentioned associated control items.
U.S. Patent No. 4,903,495, which is assigned to the same assignee as the present application, teaches the utilization of a maximum operating pressure expansion valve and a secondary condenser coil to enhance hot gas heating cycles. Refrigerant trapped in the condenser coil and receiver tank is injected into the active refrigerant flow path via the maximum operating pressure valve, when the amount of refrigerant in the active refrigerant flow path is not sufficient to build the pressure on the low pressure side of the system the point necessary to close the maximum operating pressure valve.
U.S. Patent No. 4,912,933, which is assigned to the same assignee as the present application, improved upon the arrangement of the '818 patent by connecting the receiver tank to the accumulator when the need for a heating cycle is detected, with the connection being made before the mode selecting valve means actually switches refrigerant flow to the heating refrigerant flow path. In other words, the need for a heating cycle establishes direct refrigerant flow communication between the receiver tank and accumulator while delaying the switch of the hot gas refrigerant flow from the cooling flow path, which includes the condenser coil and receiver tank, to the heating flow path. This forces refrigerant trapped in the condenser coil and receiver to flow to the lower pressure accumulator, providing an enhancement to the heating and defrost cycles. In both the '818 and '933 patents, the direct fluid flow communication between the receiver tank and accumulator is preferably maintained during the heating cycle, with a check valve preventing reverse flow into the receiver tank.
U.S. Pat. No. 4,932,219, which is assigned to the same assignee as the present application, points out that the problem of trapping refrigerant in the condenser coil and receiver tank, which is worse during low ambient conditions, is even more critical when the transport refrigeration unit is compartmentalized, ie., serving two or more separate conditioned spaces. This patent teaches the selective pressurization of the receiver tank during a heating or defrost cycle, in response to a compressor head pressure below a predetermined value, such as 200 psig. While this teaching is useful in a transport refrigeration system having a single compartment to condition, it is especially useful in compartmentalized transport refrigeration systems having two or more separate compartments to condition.
U.S. Pat. No. 5,056,324, which is assigned to the same assignee as the present application, improves upon the '933 patent by directing refrigerant from the condenser coil and receiver tank, during a time delay purge cycle, into the "heating" refrigerant path, at a point between the "heating" output port of a heat/cool mode selector valve and the evaporator coil, instead of directly into the accumulator, while the mode selector valve is still providing refrigerant to the "cooling" refrigeration path.
U.S. Pat. No. 5,157,933, similar to the '438 patent, teaches draining the condenser coil into the active refrigeration circuit, when the condenser coil becomes inactive, such as during a heating or a defrost cycle.
U.S. Pat. No. 5,172,559, which is assigned to the same assignee as the present application, teaches how to apply the teachings of the '933 patent to a compartmentalized transport refrigeration unit.
During tests of a compartmentalized transport refrigeration unit which utilized receiver tank pressurization during heating and defrost cycles, it was found that in very low ambient conditions the unit could become extremely charge sensitive. If insufficient refrigerant charge was in the active refrigerant flow path during this low ambient condition, it could result in the cooling and heating capacity of the unit slowly diminishing to the point where no cooling or heating would occur. When this happened, the temperatures in the multiple compartments would equalize. It is not desirable to start with a larger amount of refrigerant in these systems, as it increases the size and cost of the receiver and accumulator, which increases the size and cost of the unit.
Thus, it would be desirable, and it is an object of the present invention, to make maximum usage of the refrigerant charge, assuring sufficient charge in the active portion of the system at all times, for proper operation of a transport refrigeration unit, including during low ambient conditions, for both single cargo and compartmentalized transport refrigeration units.